1. Field of the Invention
The invention relates to a method for reducing degradation of fuel cell systems at transitions in operation. The invention further relates to a protector for reducing the degradation of fuel cell systems at transitions in operation. The invention moreover relates to a fuel cell system having at least one fuel cell and an electrical network connected electrically to it.
2. Description of the Prior Art
In an energy generation by means of fuel cells, in particular PEM fuel cells, increased wear at the electrodes, especially the cathode, occurs particularly as a result of transitions from one operating state to a stopped state and vice versa. In a stopped state, because of the gas-permeable membrane of the fuel cell, air and hence oxygen diffuse into the anode and cathode chambers, for lack of hermetic sealing. At the transition to the operating mode, that is, upon (re)starting the fuel cell system, fuel and in particular hydrogen is metered into the air- and hence oxygen-containing anode chamber, and as a result a hydrogen-air front develops, which propagates over the entire active surface and along the surface. At this hydrogen-air front, potential shifts occur, the effects of which range from the deactivation to the destruction of the diametrically opposed cathode in this area from oxidation of a carbon substrate, by the following equation:C+2H2−>CO2+4H++4e−, in which approximately φ00=0.207 V.
This mechanism is based on the fact that oxygen present at the anode establishes the potential equilibrium of the oxygen reaction at the phase boundary of the electrode and the electrolyte. Since the membrane potential represents the reference potential of the electrochemical electrode potential, the electrochemical potential of the anode and cathode increases accordingly. The potential increase is especially critical at the cathode, since the cathode already has a higher potential. Thus at the cathode, potentials can be reached that can amount to substantially more than 1.2 V. Such high potentials can lead to oxidation of the carbon substrate and dissolution of the existing platinum catalyst. At the anode, potentials of up to 1.0 V are reached. This potential can lead to the dissolution of ruthenium, for instance, that is present for increasing the CO tolerance. Because of the shunt conductance of the typically thin electrolyte membrane, such as a membrane made of Nafion®, the protein deficiency cannot be compensated for. The potential increase or excessive increase cannot be measured from outside, that is, between the electrodes.
After a shutoff or shutdown of the fuel cell system, (ambient) air from the cathode inlets and outlets and through the fuel cell stack seal can diffuse into the anode chamber and lead to a potential shift because of locally different oxygen concentrations.
In the prior art, principles for reducing the degradation are described. For instance, methods are described in which the cathode path is closed in airtight fashion, or in which the fuel cell system is operated galvanostatically. Alternatively, the fuel metering can be done at different speeds. All these solutions to the problem do not efficiently prevent degradation, since despite everything, potential differences are still present.